Complete the widget-in-cell gap the same value/style-convention way as the
checkbox: two more cell kinds, both plain data with a render flag.
Engine:
- CellStyle gains `markdown` and `slider` bool flags, serialized as two
trailing columns on the CELL line (older files default them off), plus
WorkbookCommand::{SetMarkdown, SetSlider} routed through apply and
apply_command via mutate_cell — undoable like every style mutation.
UI:
- markdown.rs (measured): a flat inline parser splitting a cell value into
Regular/Bold/Italic/Code runs (`**bold**`, `*italic*`, `` `code` ``);
unclosed markers and empty spans stay literal/dropped. The grid draws
each run with the bold or regular resource (code tinted like formulas).
- slider.rs (measured): 0-100 fraction/value mapping (rounded to whole
steps) and track/fill/handle geometry. A slider cell draws the control
instead of text, and a press/drag sets the value through set_cell — one
undo step per drag (reverse-order ChangeSet application restores the
pre-drag value).
- Toolbar "Md" and "Slider" buttons toggle the flags on the selection;
the render cache carries the two flags so cached cells stay styled.
Dropdown and button cells are intentionally not included: a dropdown
needs a per-cell choices list and a button needs an action semantic that
a spreadsheet does not have — both would require a CellKind in the data
model rather than a render flag.
Engine: 481 lib tests (+2) + integration. UI controllers: 133 tests (+13,
markdown 10 + slider 3). Coverage: engine 96.62%, ui-controllers 99.42%
(floors 96); markdown.rs and slider.rs at 100%.
